The present invention relates to the degassing of molten metal. Molten metal, particularly molten aluminum in practice, generally contains entrained and dissolved impurities both gaseous and solid which are deleterious to the final cast product. These impurities may affect the final cast product after the molten metal is solidified whereby processing may be hampered or the final product may be less ductile or have poor finishing and anodizing characteristics. The impurities may originate from several sources. For example, the impurities may include metallic impurities such as alkaline and alkaline earth metals and dissolved hydrogen gas and occluded surface oxide films which have become broken up and are entrained in the molten metal. In addition, inclusions may originate as insoluble impurities such as carbides, borides and others or eroded furnace and trough refractories.
One process for removing gaseous impurities from molten metals is by degassing. The physical process involves injecting a fluxing gas into the melt. The hydrogen enters the purge gas bubbles by diffusing through the melt to the bubble where it adheres to the bubble surface and is adsorbed into the bubble itself. The hydrogen is then carried out of the melt by the bubble.
It is naturally highly desirable to improve the degassing of molten metals in order to remove or minimize such impurities in the final cast product, particularly with respect to molten aluminum and especially, for example, when the resultant metal is to be used in a decorative product such as a decorative trim or products bearing critical specifications such as aircraft forgoings and extrusions and light gauge foil stock. Impurities as aforesaid cause loss of properties such as tensile strength and corrosion resistance in the final cast product.
Rigorous metal treatment processes such as gas fluxing or melt filtration have minimized the occurrence of such defects. However, while such treatments have generally been successful in reducing the occurrence of such defects to satisfactory levels, they have been found to be inefficient and/or uneconomical. Conventionally conducted gas fluxing processes such as general hearth fluxing have involved the introduction of the fluxing gas to a holding furnace containing a quantity of molten metal. This procedure requires that the molten metal be held in the furnace for significant time while the fluxing gas is circulated so that the metal being treated would remain constant and treatment could take place. This procedure has many drawbacks, among them, the reduced efficiency and increased cost resulting from the prolonged idleness of the furnace during the fluxing operation and more importantly, the lack of efficiency of the fluxing operation due to poor coverage of the molten metal by the fluxing gas which is attributable to the large bubble size and poor bubble dispersion within the melt. Further factors comprise the restriction of location to the furnace which permits the re-entry of impurities to the melt before casting, and the high emissions resulting from both the sheer quantity of flux required and the location of its circulation.
As an alternative to the batch-type fluxing operations employed as aforesaid, certain fluxing operations were employed in an inline manner; that is, the operation and associated apparatus were located outside the melting or holding furnace and often between the melting furnace and either the holding furnace or the holding furnace and the casting station. This helped to alleviate the inefficiency and high cost resulting from furnace idleness when batch fluxing but was not successful in improving the efficiency of the degassing operation itself, in that the large size of the units and the undesirably large quantities of fluxing gas required per unit of molten metal were both costly and detrimental to air purity.
A typical inline gas fluxing technique is disclosed in U.S. Pat. No. 3,737,304. In the aforenoted Patent, a bed of "stones" is positioned in a housing through which the molten metal will pass. A fluxing gas is introduced beneath the bed and flows up through the spaces between the stones in counter flow relationship with the molten metal. The use of a bed of porous "stones" has an inherent disadvantage. The fact that the stones have their pores so close together results in the bubbles passing through the stones coalescing on their surfaces and thus creating a relatively small number of large bubbles rather than a large number of small bubbles. The net effect of the bubbles coalescing is to reduce the surface area of bubble onto which the hydrogen can be adsorbed thus resulting in low degassing efficiency.
One improved method and apparatus for the inline degassing and filtration of molten metal is disclosed in U.S. Pat. No. 4,052,198 to Yarwood et al. and assigned to the assignee of the present invention. The disclosure teaches an improvement in the degassing and filtration of molten metal using an apparatus which employs a pair of sequentially placed, removable filter-type elements and at least one fluxing gas inlet positioned therebetween. The fluxing gas is introduced into the melt through the inlet and flows through the first of said plates in countercurrent contact with the melt. The filter plate serves to break up the fluxing gas into a fine dispersion to insure extensive contact with the melt. The filter plates employed are made of porous ceramic foam materials which are useful for the filtration of molten metal for a variety of reasons included among which are their excellent filtration efficiencies resulting from their uniform controllable pore size, low cost, as well as ease of use and replaceability. The ceramic foam filters are convenient and inexpensive to prepare and easily employed in an inline degassing and filtration unit.
While the aforenoted U.S. Pat. No. 4,052,198 offers significant improvements over those inline gas fluxing techniques previously known in the art, a number of problems have been encountered. It is desirable for economic advantages and increased productivity to have degassing and filtration systems which can treat molten metal continuously at a rate commensurate with the casting practices. The employment of known inline degassing units such as aforenoted U.S. Pat. No. 3,737,304 for continuous degassing and filtration have been found to be extremely inefficient, thus requiring large multiple chamber arrangements necessary to sufficiently treat the quantities of molten metal which are required for continuous casting operations. As a result of the large size of the treatment units, supplemental heating is required to prevent freeze up the molten metal as it is being treated. While some improvement in the quantity of molten metal which can be treated has been achieved by using a smaller system such as that disclosed in U.S. Pat. No. 4,052,198 which utilizes ceramic filters and countercurrent gas flow, such a system has been found to have a limited effectiveness in the quantity of molten metal which can be treated due to the large pressure drops encountered in the simultaneous countercurrent flow of gas and metal through the filter body. As a result of the large pressure drop, a large head of molten metal is developed upstream of the filter element thus requiring either an increase in size of the transfer passage-way upstream of the filter element or a decrease in the rate of feeding the molten metal to the treatment unit.
Accordingly, it is a principal object of the present invention to provide an improved method and apparatus for the degassing and filtration of molten metal which employs filter-type plates which are characterized by reducing the pressure drop encountered in the simultaneous countercurrent flow of gas and metal through the filter-type plates.
It is a particular object of the present invention to provide an improved filter-type plate for reducing the pressure drop encountered across the filter plate as gas and metal flow in countercurrent relationship through the filter-type plate.
It is still a further object of the present invention to provide a filter-type plate having a preferential path for gas flow through the filter plate.
It is still a further object of the present invention to provide improvements as aforesaid which are convenient and inexpensive to utilize and which result in highly efficient metal degassing and filtration.
Further objects and advantages of the present invention will appear hereinbelow.